This invention relates to a pressure balanced annular seal. In particular the invention concerns a segmented annular seal assembly for sealing between relatively movable components in a gas turbine engine.
The invention has for a principal objective the reduction or elimination of pressure loading on seal segments in the direction of main gas flow by use of pressure balance features acting in the opposite direction to the main direction of gas flow.
The relatively movable components of the present invention are commonly relatively rotatable, but in this case movable is intended to embrace lateral or transverse movement as well as rotational movement. It will be understood that, in use, a turning shaft, rotor or the like may also be subject to a certain amount of lateral or transverse movement. Therefore it will be understood also that such a seal will be effective even when the shaft rotor or the like is rotationally stationary.
A segmented annular seal assembly is known from U.S. Pat. No. 6,669,203 in which a plurality of arcuate brush seal segments are disposed in a radially inwardly opening groove of a stationary component to engage the surface of a relatively rotating component. Each brush seal segment has limited radial and axial clearances with respect to the fixed housing provided by a hook or flange carried on the upstream side of each segment engaged with a slot formed in the housing. A spring is disposed between the segment and the base of the groove and biases the seal segments towards the rotating component so as to follow radial movements. It is claimed that this arrangement permits at start up high pressure flow on the upstream side to bias the segments in an axial downstream direction and also permits the high pressure to enter into the base of the groove and the radially outer face of the segments to bias the segment radially inwardly to ensure proper sealing of the bristle tips along the rotor surface.
However, a disadvantage of the arrangement is that friction between the downstream faces of the segments and the downstream wall of the groove due to the high pressure on the upstream side can cause the segments to stick to the wall and fail to track radial excursions of the rotor leading to increased seal leakage. This can lead to uneven and excessive seal wear, and it is impracticable to employ a bias force sufficient to overcome the sticking force as it would increase seal wear to unacceptable levels.
In U.S. Pat. No. 6,572,115 a segmented seal is mounted in a seal carrier which is movable in radial directions to accommodate radial transients. The seal carrier segments are disposed in a channel creating a space on the radially outer side of the segments to which upstream pressure is supplied thereby create a bias force acting radially downwards to bias the seal portions into contact with the rotor. However, the structure of the embodiments disclosed exposes the upstream facing faces of the movable seal parts to upstream pressure, and the downstream facing faces of the movable seal parts to downstream pressure. Consequently the seal carrier segments are subject to a tilting force as a result of the difference in pressure between the upstream and downstream sides. This may lead to the segments jamming in the channel so that they cannot follow radial transient movements and suffer uneven and premature wear as a result.
In such prior art arrangements as these shaft seal segments which slide (radially in the case of axial flow, axial in the case of radial flow) suffer from high contact loading associated with the net pressure differential across them. The imbalance in pressure across the seal causes a net force in the direction of fluid flow. The segments then seize against the retaining plate/ring and segment movement is either very difficult or prohibited. The additional forces required to move the segments manifest themselves as higher loading between the seal and a relatively rotatable part or a static structure, in the case of a static seal. Where there is relative movement high wear results.